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The purpose of this study was to evaluate the effect of a low amount of torsional preloading on the fatigue life and different degrees of cyclic fatigue on torsional failure of HyFlex EDM (EDM; Coltene-Whaledent, Allstetten, Switzerland) and HyFlex CM (CM; Coltene-Whaledent) instruments.EDM and CM files were used. The fatigue resistance was examined in a 5-mm radius and 60° single curve, and the mean number of cycles to failure (Nf) was recorded. The torque and rotation angles at failure of the instruments were measured according to ISO 3630-1. New files were precycled to 0%, 50%, and 75% of the Nf, and torsional tests were then performed. Other new files were preloaded at 5%, 15%, 25%, and 50% of the mean rotation angles before the fatigue test. The fracture surfaces of the fragments were examined under a scanning electron microscope.The fatigue resistance of EDM instruments was higher than that of CM instruments (P < .05). The torque and angle of rotation at fracture of the files were similar. Torsional preloading lowered the Nf of EDM at 15% preloading (P < .05) and the Nf of CM at 50% preloading (P < .05). However, the Nf of EDM files even with 50% torsional preloading was significantly higher than unused CM files (P < .05). Fatigue prestressing even at 75% had no negative effect on the torque and rotation angle of the EDM files. Moderate precycling (50%) of EDM files increased their torsional resistance. The fractographic patterns corresponded to the pattern defined by the last stage test.A low amount (15%) of torsional preloading reduced the fatigue resistance of EDM files, whereas even extensive (75%) precyclic fatigue was not detrimental to their torsional resistance.The effect of a low amount of torsional preloading on the fatigue life and various degrees of cyclic fatigue on torsional failure of HyFlex EDM are evaluated.The fatigue resistance of EDM files is higher than that of HyFlex CM files.A low amount of torsional preloading reduces the fatigue resistance of EDM files.Even extensive precyclic fatigue would not be detrimental to the EDM file's ability to withstand the torsional load.